1. The Field of the Invention
The present invention relates to x-ray tubes having rotating anode structures. In particular, embodiments of the present invention relate to structures and assembly methods for a rotor shaft and rotor body assembly of an x-ray tube rotating anode.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing.
The basic operation of typical x-ray producing equipment is similar. In general, x-rays, or x-radiation, are produced when electrons are released, accelerated, and then stopped abruptly. A schematic representation of a typical x-ray tube is shown in FIG. 1. The illustrated x-ray tube assembly 1 includes three primary elements: a cathode 2, which is the source of electrons; an anode 3, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and a voltage generation element for applying a high voltage potential to accelerate the electrons from the cathode to the anode.
The three elements are usually positioned within an evacuated housing 4. An electrical circuit is connected so that the voltage generation element can apply a high voltage potential (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The voltage differential causes the electrons that are emitted from the cathode 6 to form a beam and accelerate towards an x-ray "target" that is positioned on the surface of a anode disk 5. The target surface (sometimes referred to as the focal track) is comprised of a refractory metal, and when the electrons strike the target at the focal spot, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target, and are then collimated through a window 9 for penetration into an object, such as an area of a patient's body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
In addition to producing x-rays, when the electrons impact the target surface much of the resulting energy is converted to heat. This heat, which can reach extremely high temperatures, is initially concentrated in the anode target and then dissipated to other areas of the x-ray tube. These high operating temperatures can damage the x-ray tube, especially over time.
The anode disk 5 (also referred to as the rotary target or the rotary anode) is rotatably mounted on a rotating nose piece or stem and rotating shaft 11, which is connected to a supporting rotor assembly 7. The disk 5, shaft and rotor assembly are rotated by a suitable means, such as a stator motor 8. The disk is typically rotated at high speeds (often in the range of 10,000 RPM), thereby causing the focal track to rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with specific points along the focal track for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes the portion to rotate back into the path of the electron beam.
It will be appreciated that the need to continuously accelerate and rotate the disk at such high speeds in the presence of extremely high temperatures can give rise to a number of problems. For instance, while the rotation of the track helps reduce the amount and duration of heat dissipated in the anode target, the focal track is still exposed to very high temperatures--often temperatures of 2500.degree. C. or higher are encountered at the focal spot of the electron beam. This heat is transferred to other portions of the x-ray tube assembly, including the shaft and rotor assembly, resulting in extreme thermal stresses at the interfaces between the various structures. Moreover, acceleration and deceleration of the relatively heavy anode disk results in severe mechanical stresses being imposed on the rotor assembly. Unfortunately, the structures and assembly methods used for anode disk rotational assemblies have not been entirely satisfactory in addressing the various problems arising from such mechanical and thermal stresses.
For example, a rotor shaft and rotor body assembly have typically been interconnected by way of threads formed on an outer portion of the rotor, which is then received within a corresponding threaded bore formed within a portion of the rotor body. In addition, a brazed joint may be applied between the threaded mating surfaces. Also, a screw, pin, or the like may be used to secure the rotor shaft to the rotor, which assures that the rotor shaft does not detach from the rotor body in the event that the threaded engagement/braze joint fails. Finally, the rotor shaft may be further welded to rotor body by use of an electron beam welding method.
It will be appreciated these types of manufacturing steps are time consuming, expensive, and can result in an assembly with multiple points of potential failure. For instance, the formation of a threaded rotor shaft and corresponding mating rotor body, along with the placement of a screw or the like, entails intensive machining and assembly. Additionally, the placement of a screw or similar fastening means may itself be an operation that is subject to occasional defects. Also, electron beam welding can cause brittleness at the weld that may lead to structural failure, which is made even more likely due the extreme temperature fluctuations that are encountered during operation of the x-ray tube. Finally, each of these techniques entail expensive and time consuming manufacturing steps, which increase the overall production cost of the x-ray tube device.
The types of materials that are typically used in the construction of a rotor shaft and rotor body can also give rise to problems. For instance, to restrict the flow of heat by conduction into the rotor shaft and rotor body assembly from the rotating anode target disk, the rotor shaft is often provided with a minimum cross-sectional size and is generally made of a relatively poor heat conductive material, such as a molybdenum alloy called TZM. TZM comprises about 99% molybdenum with the balance making up various proportions of titanium and zirconium. While the TZM material exhibits superior structural strength, it can have a different linear coefficient of thermal expansion than the material making up rotor body 14. For instance, the rotor body is often made of an iron alloy such as Incoloy 909 sold by Inco Alloys International Inc. of Huntington, W. Va., which has a linear thermal expansion coefficient that is slightly different from that of TZM. This can give rise to significant structure-weakening events during operation, due to the varying rate of thermal expansions of the two materials.
Also, where the rotor body is constructed of iron or an iron alloy material, the extreme temperature fluctuations can cause such an iron-based alloy to experience allotropic transformation from body centered cubic (bcc) to face centered cubic (fcc). For instance, when rising through about 912.degree. C., iron transforms from bce to fcc and consequently shrinks in volume. Therefore, in addition to disparate linear thermal expansion coefficients, allotropic transformations cause additional stress upon a braze joint at the interface between rotor shaft and rotor body.
Many of these problems can be manifested during repeated operation of the x-ray tube. During operation, the rotor shaft begins to heat up and mechanical stresses from high rotational speeds are imposed. For instance, when the rotor shaft is connected to the rotor assembly with a threaded interface and a braze joint, a horizontal thermal shear plane is often produced at the threaded interface between the shaft and the rotor body within the braze joint. This thermal shear stress can be transferred through the braze material. Moreover, the condition is exacerbated if rotor body 14 is made of iron or an iron alloy, and is taken through the allotropic transformation temperature threshold of about 912.degree. C., as noted. Over time, this continuous cycle of expansion and contraction can result in a cracks or other failure points in the joint. Once a crack has nucleated, propagation of the crack typically results, ultimately resulting in failure of the x-ray tube.
Other problems can also result when traditional methods are used to interconnect the rotor assembly. For instance, the braze joint is often comprised of a braze material that will readily flow along and between the threaded surfaces of the rotor shaft and the rotor body. In the event that the braze material has a melting temperature above 1150.degree. C., the molybdenum component of the TZM material forming the rotor shaft forms a eutectic with the metal component of the brazing material, that in turn produces an intermetallic compound. This compound can be brittle in comparison to most metals at room temperature, and can become more ductile as the temperature increases, where conventional metals may tend to allotropically transform and fail or even reach liquidus temperatures. Alternatively, if the braze material has a melting temperature below about 900.degree. C., the braze joint may soften during operation of the x-ray tube and fail to withstand the resulting mechanical stresses.
Thus, what is needed is a rotor shaft and rotor body assembly that overcomes the problems of the prior art. In particular, it would be advantageous to have a rotor shaft and rotor body assembly that are interconnected in a manner so as to better withstand the extremely high temperatures and mechanical stresses imposed during operation of the x-ray tube. Additionally, it would be advantageous to provide a rotor shaft and rotor body assembly that are interconnected in a manner so as to resist cracking within the braze joint. Also, it would be advantageous to provide a interconnection scheme that is easy and low in cost to implement and manufacture.